With the recent progress of miniaturization of electronic and electric equipment to higher density, the problem of heat dissipation of heat-generating electronic parts such as power transistors built in such equipment becomes highlighted.
Among heat-generating electronic parts used in the art, for example, the power transistor has a structure as shown in FIG. 1. A power transistor T includes a power transistor body 3 of rectangular shape having a bottom surface 3a and a top surface 3b which are flat, a plurality of (three in FIG. 1) terminals 5 projecting from one end of the body 3, and a plate-shaped heat spreader 4 attached to the transistor body 3. Specifically, a lower corner portion of the transistor body 3 opposed to the terminal-projecting end is machined in a quadrangular shape to define a notch 3c. One end of the plate-shaped heat spreader 4 is fitted in the notch 3c while the other end of the heat spreader 4 extends in a direction opposite to the terminal projecting direction. The plate-shaped heat spreader 4 having a flat bottom surface is attached to the transistor body 3 such that the bottom surface of the heat spreader 4 is flush with the bottom surface 3a of the transistor body 3. Any one of the three terminals 5 extends through the power transistor body 3 and is connected to the plate-shaped heat spreader 4. Typically, the power transistor T of the above-described structure is fixedly secured to an external heat sink (not shown) such as a metal chassis while an electrically insulating, heat transfer sheet of synthetic resin or rubber is interposed between the external heat sink and the heat spreader 4. The heat produced by the power transistor T during operation is conducted from the heat spreader 4 to the external heat sink. Heat dissipation occurs in this way.
When sheets of synthetic resin or rubber were interposed, there arose problems including inefficient operation due to frequent misalignment and too large a creepage distance for electrical insulation. It was thus proposed in JP-U-A S57-2666 to use a tube of synthetic resin or rubber. FIG. 2 illustrates a tubular cover as disclosed therein. The tubular cover in FIG. 2 is cylindrical and open at both upper and lower ends 1 and 2.
When the power transistor is enclosed using the tubular cover, the power transistor with its plate-shaped heat spreader 4 facing forward is inserted into the tubular cover through the lower open end 2 until only the three terminals project out of the lower open end 2 of the cover. Typically the cover having the power transistor accommodated therein is fixedly secured to an external heat sink. In the event that the power transistor is enclosed within a prior art tubular cover, surplus spaces are left around the power transistor because the cross-sectional shape of the tube does not conform to the cross-sectional shape of the transistor. The presence of such surplus spaces is against the trend toward miniaturization of electronic equipment.
Where the power transistor is accommodated within the tubular cover, the rear end of the tubular cover opposite to the terminal-projecting one end is left open. Now that electronic parts are assembled at a high density, an electric discharge can occur between the heat spreader and an external heat sink or another electronic part through this rear opening, causing a malfunction.
Then, as shown in FIG. 3, an open rear portion of the tubular cover 6 is folded along an inside line over the remaining portion, closing the rear end of the cover. The end-folded cover 6 is interposed between two external heat sinks 7 and 8, which are secured together by a screw 9. This overcomes the problem associated with a tubular cover having open ends.
When the tubular cover is set in place within an electronic equipment in this way, however, the folded portion is often stretched out and the folded portion provides a thickness buildup to prevent smooth attachment, adversely affecting operation efficiency. Undesirably the cover tends to start degradation from the folded portion.
One solution to these problems is shown in FIG. 4. JP-U-B H03-53510 discloses a cover for a heat-generating electronic part, comprising a heat conductive, electrically insulating sleeve 12 which is open at one end 10 and closed at another end 11. The width and height of the opening substantially correspond to the width and height of an electronic part to be inserted therein. The sleeve 12 has flat inside surfaces that define an internal space and correspond to the flat surfaces of the electronic part. The cover 12 of this utility model has advantages including easy attachment, little increase in the overall volume of the power transistor as a result of cover attachment, and a reduction in creepage distance to another electronic part or external heat sink, but suffer from problems as discussed below. The cover includes a top wall 12a and a bottom wall 12b of substantially the same thickness having flat inside surfaces corresponding to the flat surfaces of the electronic part. The cover 12 having the electronic part T enclosed therein is secured to external heat sinks such as heat-dissipating metal fins. If the cover top and bottom walls 12a and 12b in contact with the external heat sinks are too thick, the cover exerts poor heat-dissipating effect. Inversely, if the cover top and bottom walls 12a and 12b are too thin, the cover may allow for short-circuiting with the heat-dissipating metal fins or exerts poor electrically insulating effect. The same cover with top and bottom walls of equal thickness is not applicable to both an application where heat dissipation is a matter of concern and a power supply or similar application where electrical insulation is a matter of concern. Plural types of covers which differ in the thickness of top and bottom walls (for example, one type of cover having thick top and bottom walls and another type of cover having thin top and bottom walls) must be furnished in advance for the respective applications. This causes such inconvenience as a need for capital investment for a plurality of molds.